Why is preliminary work important for a science experiment ?

Preliminary work (sometimes called a pilot experiment) can establish the effectiveness of a particular protocol. Also, such preliminary trials or measurements can give the researcher an indication of the amount of variability among subjects or the repeatability of measurements on the same subject, which will then allow them to calculate the number of replicates they should use in the main experiment to achieve a given level of precision or power (both from a statistical point of view). Finally, preliminary work/observations can lead to further questions about a given system.